Energy transfer in restricted geometry: Polyisoprene-poly(methyl methacrylate) block copolymer interfaces

The kinetics of direct nonradiative energy transfer between dyes confined to the 2.6 nm wide interface region of polyisoprene-poly(methyl methacrylate) block copolymer films are reported. This system differs from restricted geometry systems examined previously because of the diffuse nature of the edges of the confining space. The interface thickness is similar in magnitude to the characteristic distance for energy transfer (R(0) = 2.3 nm) for the donor-acceptor dye pair (phenanthrene-anthracene) employed here. Samples were prepared from matched pairs of block copolymers, one containing a donor dye and the other an acceptor dye, at the PI-PMMA junction. Donor fluorescence decay profiles were fitted to the Klafter-Blumen expression [I-D(t) = A(1) exp{-(t/tau(D)) - P(t/tau(D))(beta)} + A(2) exp(-t/tau(D))] containing the additional A(2) term to account for donors (ca. 3%) outside the interface. The parameters obtained followed the predicted behavior, namely, that the preexponential term P was proportional to the acceptor concentration, whereas the stretched-exponential parameter beta was independent of the global acceptor concentration C-A for acceptor-to-donor ratios C-A/C-D > 1. One of the most unusual features of the data is a crossover in beta observed as a function of a global acceptor concentration C-A for a certain range of donor-acceptor composition, C-A/C-D < 1.